Recently, it has become popular to process and manage the sound and video in digital format in appliances such as acoustic appliances and video appliances. Such trends in digital encoding of sound and video in appliances such as acoustic appliances are extending to the field of radio broadcasting. For example, in the United States, a digital radio broadcasting system called IBOC (In Band On Channel) is proposed and made available by iBiquity Digital Corp.
Meanwhile, popular conventional analog radio broadcasting broadcasts via carrier wave (Hereinafter, “analog carrier wave”.) that has frequency distribution inside the frequency band corresponding to physical channel (Hereinafter, “channel” or “frequency channel”.) assigned to individual broadcasting stations. Actually, in order to avoid the interference between analog carrier wave of adjacent channels, only the center portion of the assigned band is used for the transmission of the analog carrier wave, and other portions are not used. It is noted that “digital radio broadcasting” in this application means “IBOC digital radio broadcasting”.
IBOC is a type of digital radio broadcasting that uses frequency channel assigned to the conventional analog radio broadcasting. In IBOC standard, a plurality of signal formats are defined, such as hybrid format in which the digital radio broadcasting signal is multiplexed onto the conventional analog radio broadcasting signal, and all-digital format consisted of only digital signals, and it is designed to gradually transfer from conventional analog radio broadcasting to all-digital radio broadcasting that has many functions and is high in quality. In the IBOC, digital broadcasting signals are transmitted with Orthogonal Frequency Division Multiplexing (OFDM) that uses many carrier waves (subcarriers).
In contrast, in the IBOC standard, signal format called “hybrid format” is used in the transition period from analog broadcasting to all-digital broadcasting. In the hybrid format, the digital radio broadcasting, which allocates the subcarriers of digital broadcasting in the portion that is adjacent to the center portion of the band that the analog carrier wave uses and that was not conventionally used (Hereinafter, “sideband”.) is broadcast using the modulated wave of the sideband of the band. In other words, in accordance with the hybrid format of the IBOC, the frequency band for the conventional analog radio broadcasting is utilized effectively, and the analog radio broadcasting and the digital radio broadcasting are simultaneously transmitted using a same channel.
For example, Japanese Patent Provisional Publication No. JP2004-349805 (hereinafter, “the Reference Document”.) discloses an IBOC broadcasting receiver that is capable of receiving such IBOC digital radio broadcasting. First, the IBOC broadcasting receiver disclosed in the Reference Document filters the received signal to pass wide frequency band that includes a center portion in which the analog carrier wave of the selected frequency channel is located and the portion (sideband) in which the adjacent subcarriers are located, and the subcarrier as well as the analog carrier wave is decoded. Then, if IBOC signal (identification information showing that it is digital radio broadcasting) is acquired as a result of the decoding, since the digital radio broadcasting is transmitted in the selected frequency channel, the setting of the filtering is maintained to pass a wide range of band.
On the other hand, if IBOC signal was not acquired, then only analog radio broadcasting is transmitted in the selected channel, and no valid information is included in the sideband. In addition, not only the sideband does not include valid information, it easily suffers from disturbances due to the adjacent band (It is the noise, etc. Hereinafter, it is called “adjacent disturbance”.), and it becomes the cause of the lowering of the carrier-to-noise ratio (CN ratio) of the selected frequency channel. Therefore, in case the IBOC signal is not acquired, then the signal of the selected channel is filtered to pass the bandwidth of the analog carrier wave. Thereby, the sideband that is unneeded and easily affected by the adjacent disturbance is cut off, and the CN ratio for the analog radio broadcasting is improved.
That is, the IBOC broadcasting receiver disclosed in the above Reference Document is arranged to improve the CN ratio of the analog radio broadcasting so that the band to pass the filter is enabled to be switched according to the existence of the IBOC signal, and the sideband is cut off to improve the CN ratio only if it is determined that the selected channel includes only analog radio broadcasting.